The particular feature of the EUV masks is that they are used in reflection mode and not in transmission mode. They are reflective for the useful EUV wavelength, that is to say the one which will be used for the photolithography operations with this mask. The binary EUV masks also relate to a pattern of zones that are absorbent for the useful EUV wavelength. EUV masks with phase offset relate to a pattern of phase-shifting zones. To simplify the explanations, it is hereinafter considered that the masks are binary masks, although the invention applies also to masks with phase offset.
In use, the mask is lit by an EUV light and reflects this radiation, except in the absorbent zones where the light is absorbed and cannot be returned. The EUV lighting has a well determined wavelength and is spatially modulated by this pattern and is projected by a focusing optic with mirrors onto a surface to be exposed. The surface to be exposed is a layer of EUV-sensitive resin, deposited on a planar substrate. This layer covers the layers that are to be etched or treated (for example implanted) after the exposure of the resin to the EUV radiation. The subsequent chemical development of the resin leaves a structure in which the layers to be etched or to be implanted are covered with a resin pattern which protects certain zones and reveals other zones.
The projection optic reduces the image and makes it possible to define, in the resin, smaller patterns than those which are etched on the mask. The reduction ratio is generally 4. The mask is generally fabricated from an electron beam writing method.
Typically, a reflection mode mask of binary mask type consists of a planar substrate with low expansion coefficient, covered by a reflecting structure; the reflecting structure is, more often than not, a Bragg mirror, that is to say a structure with multiple transparent layers of different refractive indices. The thicknesses of these layers are computed as a function of the indices, of the wavelength, and of the angle of incidence of the EUV beam, so that the different interfaces, partially reflecting, return lightwaves in phase with one another. The mirror is covered by an absorbent layer etched according to the desired masking pattern, so that the mask comprises reflecting zones (the mirror not covered by absorbent) and absorbent zones (the mirror covered by absorbent). As an example, for a wavelength of 13.5 nm and an angle of incidence of 6 degrees, some forty or so layers of silicon 41.5 angstroms thick (1 angstrom=0.1 nm) will be used, alternated with some forty or so layers of molybdenum 28 angstroms thick. The absorbent zones can be made of chrome (among others) deposited on the mirror; for example, a 600 angstrom layer of chrome placed on the mirror above reflects no more than 1% of the incident light.
The substrate bearing, on its entire surface, a multilayer mirror and a uniform (therefore not yet etched) absorbent layer is called “mask blank”. The mask blank is etched according to a desired pattern to form an EUV photolithography mask. The small size of the masking patterns to be produced by the EUV photolithography means that defects of the mask blank can result in defects that are damaging to the photolithographed structure. Small defects of a few tens of nanometres in dimension on the mask can be translated into undesirable patterns that can culminate in unusable structures.
The defects of the mask blank can originate from defects on the surface of the mask blank, or even from defects introduced during the formation of the multiple layers of the Bragg mirror, or finally from the surface defects of the underlying substrate itself, such as scratches, holes, bosses, defects which are propagated in the multilayer structure and are like the mirror defects. The defects are defects of amplitude (absorbent zones which ought not to be absorbent, or vice versa), or defects of optical phase (introduction of an unwanted phase shift when the photolithography light penetrates into the layers of the mask, locally damaging the reflection coefficient).
To give an order of magnitude one objective is to produce a mask that has a number of defects having a size of 60 nanometre or greater that is less than 0.01 defect per cm2. However, the existing technologies do not as yet make this possible.
It has already been proposed to correct the defects as follows: an individual mapping of the defects of each mask blank that is to be used to fabricate the series of masks necessary to the production of a structure (for example a semiconductor wafer bearing multiple microelectronic circuits) is produced. A number of masks are required, corresponding to the different levels of etching or implantations to be produced on the structure. The defects of a series of mask blanks are detected by using market-standard equipment, the position and the size of the defects on each mask blank are noted.
Software determines which mask blanks are usable for the different masks, on the basis of the implantation or “layout” diagrams of the different levels of the circuit to be produced, by providing small X or Y axis offsets or small rotations of the masks so that any defects of the mask blanks are moved outside the designs of the structure (at least outside the most critical zones of the designs).
In the case where the number of defects per mask remains high, it is difficult by this method to find a solution which culminates in them all being placed in an absorbent zone, because there is a low probability of the different defects of a mask all being able to be located in non-critical places when there are only two degrees of freedom in X, Y translation in the plane of the mask and one degree of freedom in rotation in this plane.